What happens after I open the chart editor?
A chart creation / editor wizard will appear, where the steps are already pre-filled with the values that were provided when the chart was created.
The editing steps are the same as the steps used to create the chart, as explained here.
Are chart edits updated everywhere the chart is used?
Yes — when you finish editing and save the chart, it is updated on all posts or pages where it has been inserted.
When you finish editing, and save the chart, it will be updated on all posts or pages where it has been inserted. If you want charts that also react to visitors filtering a table on the same page, learn how to make charts follow table filtering.